Our roof was replaced after a hail storm in 2010 by an out-of-state contractor who worked on several homes in our neighborhood. After a few years I noticed black mold growing on the underside of the plywood on the back side of roof. The front side has no mold. The sun rises in the front of home and sets in the back. I believe the home faces southeast. A few contractors have said it may be a ventilation isse, but that hasn’t changed in the last 23 years we owned the home. I notice water drips on the nails in winter that have come through the plywood and that the plywood is damp on the back side of house. I had a mold remediation company give me an estimate to rid the mold, but am concerned that it will grow back unless the root of the problem is discovered. Could it be no tar paper used on the back side? The company that did the roof said they would send someone over but they never did.
It is strictly and entirely due to lack of ventilation. The antiquated vents are probably at each peak of the gables, not enough to release water vapor that builds upon in the attic. This water vapor condenses on cool spots, on the shaded parts of the roof. Sunny sides of the roof are not affected because of the heat generated by the sun. The lack of tar paper may be allowing release of a bit of water vapor through the roof, but not enough to change things. What you must do: Install a ridge vent, one that will allow moist air to escape through the ridge vent that goes the entire length of the roof’s ridge. Then install soffit vents on each side of the roof in the soffit, which is the under part of the roof overhang, on each side of the roof. The best soffit vent is a 2-inch-wide screened strip going the full length of each soffit.